A thermostat is provided in a cooling water flow passage of an internal combustion engine system. The thermostat closes a water passage to a radiator when an engine is cold. By doing so, cooling water is not circulated to the radiator. As a result, the warm-up of the engine is promoted. When the temperature of the cooling water reaches a preset valve opening temperature of the thermostat, the thermostat adjusts a flow rate of the cooling water to the radiator by opening the water passage to the radiator. As a result, the cooling water is maintained at a suitable temperature.
If the thermostat breaks down, a control as described above cannot be executed. Accordingly, JP2004-316638A proposes a technique for determining a failure of a thermostat. In this JP2004-316638A, the failure of the thermostat is determined if a detected engine water temperature is lower than a reference determination temperature calculated in consideration of the influence of traveling wind.